Remove psql's ancient hack that suppressed functions taking or returning

Remove psql's ancient hack that suppressed functions taking or returning
cstring from the output of \df.  Now that the default behavior is to
exclude all system functions, the de-cluttering rationale for this behavior
seems pretty weak; and it was always quite confusing/unhelpful if you were
actually looking for I/O functions.  (Not to mention if you were looking
for encoding converters or other cases that might take or return cstring.)